Separation of AM cochannel signals in an overloaded signal environment

ABSTRACT

Multiple cochannel AM signals received in an overloaded signal environment may be separated using RF data through a method of iterative projections.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 10/930,732 filed Aug. 31, 2004, the disclosure ofwhich is expressly incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to radio frequency (RF) signalreception, and more particularly to reception and separation ofcochannel amplitude modulated (AM) signals.

2. Description of the Related Art

Cochannel signal interference occurs when two or more signals aretransmitted at the same time over the same frequency range. For example,cochannel signal interference may be encountered by a receiver that isreceiving two or more signals transmitted at the same frequency and atthe same time by two or more separate transmitters. In such a case, data(e.g., voice data, text data, etc.) contained in any one of theinterfering cochannel signals cannot be accessed or processed furtherwithout first separating the given signal from the other signals toallow demodulation or other further signal processing.

In the past, beamforming and interference cancellation techniques suchas spatial interference cancellation have been employed for purposes ofcochannel signal separation. These techniques employ multiple sensors toseparate a given signal of interest by canceling or nulling out othercochannel signals from the signal of interest. However, such approachesrequire spatial separation of sources in addition to expensive coherentmulti-channel tuners having a number of channels corresponding to anumber of sensors that is equal to or greater than the number ofcochannel signals. When the number of signals exceeds the number ofsensors, the signal environment may be characterized as overloaded.Performance of traditional beamforming and interference cancellationtechniques typically fails or degrades in such an overloaded signalenvironment.

SUMMARY OF THE INVENTION

Disclosed herein are methods and systems that may be implemented toseparate cochannel AM signals in an overloaded signal environment, i.e.,a signal environment where the number of cochannel signals exceeds anumber of separate sensors (e.g., separate antennas). Advantageously,the disclosed methods and systems may be implemented in one exemplaryembodiment to achieve separation of cochannel AM signals in anoverloaded environment using data obtained from one sensor through amethod of iterative projections. When implemented to use data obtainedfrom a single channel tuner, for example, the iterative methodology ofthe disclosed methods and systems eliminates the need for multi-channelcoherent hardware and the constraint of spatially separated multiplesources.

In one embodiment, the disclosed methods and systems may be implementedto improve reliability and performance of an AM receiver system byenabling cochannel signal separation even when the number of AMcochannel signals exceeds the number of sensors or antenna elements. Inanother embodiment, the disclosed methods and systems may be implementedto reduce the cost, complexity and/or size of an AM receiver system byreducing the number of sensors and corresponding separate tuner channelsthat are required to separate cochannel AM signals. In yet anotherembodiment, the disclosed methods and systems may be implemented toallow more efficient use of the AM frequency band by allowing selectionand separation of one or more target AM signals from a number ofcochannel AM signals that are intentionally transmitted on the samefrequency, and potentially allowing simplification or elimination of thegovernment approval process for establishing new AM broadcast channels.

As an example, in one exemplary embodiment, the disclosed methods andsystems may be implemented as a receiver system that is capable ofseparating or isolating at least one AM signal from two or moretransmitted cochannel AM signals using a single sensor (e.g., singleantenna) coupled to a corresponding single channel tuner. Such asingle-sensor implementation may be utilized to achieve cost savings andreduced receiver system size, facilitating installation of such areceiver system on mobile platforms (e.g., ships, aircraft, automobiles,trains, unmanned aerial vehicles, model aircraft, etc.), whereimplementation of larger and more costly multi-sensor antenna receiversystems are impractical or impossible.

The disclosed methods and systems may be advantageously implemented inone embodiment to selectively isolate (e.g., for purpose of listening,further processing, etc.) one or more desired cochannel AM signals fromthe cochannel AM signals that have been separated from an overloadedsignal environment that is unintentionally or undesirably created, e.g.,such as when a permanent or mobile receiver is geographically positionedbetween two transmitters that are transmitting AM signals over the samefrequency at the same time. One example of such a situation isvehicle-based receiver that is located between two cities havingtransmitters that are simultaneously broadcasting AM signals at the samefrequency.

However the disclosed methods and systems may also be implemented inanother embodiment to enable selective isolation of one or more AMsignals that have been separated from an intentionally or deliberatelycreated overloaded signal environment. In this regard, two or more AMsignals may be intentionally transmitted at the same time over the sameselected frequency range in manner to more efficiently utilize theselected frequency range. In such an embodiment, the cochannel AMsignals may originate or be transmitted in any manner suitable forcreating an overloaded signal environment from which the AM cochannelsignals may be separated and at least one of the AM cochannel signalsmay be isolated using the disclosed methods and systems. For example,the cochannel AM signals may originate or be transmitted fromgeographically remote locations (e.g., by transmitters and antennaslocated in separate adjacent cities, by transmitters and antennaslocated in different geographical areas of the same city, etc.), and/orthe cochannel AM signals may originate or be transmitted from a commongeographic location (e.g., by transmitters and antennas located at thesame radio station or other facility).

In one exemplary embodiment, multiple commercial AM radio signals may beintentionally transmitted over the same selected frequency range. Inanother exemplary embodiment, AM radio signals that contain publicservice information (e.g., weather-related information, highway-relatedinformation, emergency broadcast system “EBS” information, etc.) may beintentionally broadcast either continuously or on an as-needed basisover the same selected frequency range used by, or that may be used by,other transmitter/s of AM signals (e.g., commercial AM radiotransmitters). For example, the disclosed methods and systems may beimplemented to allow intermittent public service broadcasts (e.g., uponoccurrence of a catastrophic event such as plane crash, earthquake,tornado, hurricane, etc.) to be transmitted over one or more AMfrequencies (e.g., over a selected number of multiple AM frequencies)that may be shared by local commercial AM radio stations. In such anembodiment, a receiver may be configured according to the disclosedmethods and systems to isolate the public service broadcast from othercochannel AM signals.

Whether an overloaded signal environment is intentional or not, thedisclosed methods and systems may be implemented in one embodiment inspecialized public service radios that are designed to isolate a publicservice AM broadcast signal from an overloaded signal environment if itshould happen to exist at time of the public service transmission (e.g.,to help ensure that the public service transmission is received evenunder adverse cochannel signal conditions). In another embodiment thedisclosed methods and systems may be implemented as part of a commercialAM radio receiver that is configured to receive commercial radiobroadcasts under normal operating conditions, but that is alsoconfigured to isolate and identify intermittent public service broadcastsignals when they occur in an overloaded signal environment. Such areceiver may be optionally configured to preferentially play the publicservice broadcast to a listener. In any case, a signal environment maybe overloaded prior to the public service transmission, or may becreated by virtue of the transmission of the public service transmissionsimultaneous to other AM signals on the same frequency (intentionally orunintentionally).

In any case, selective isolation of a given cochannel AM signal fromother cochannel AM signals that have been received and separated from RFdata received in an overloaded signal environment may be performed inresponse to a command specifying the identity of the given one of thecochannel AM signals. Such a command may originate, for example, fromany source suitable for selectably choosing a given cochannel AM signalfor isolation, e.g., a human user choosing a desired cochannel broadcastfor listening, a computer processor choosing a selected cochannelbroadcast needed for performing a specific task at hand, etc.

In one respect, disclosed herein is a method for processing AM signalsthat includes: receiving RF data in an overloaded signal environment,the received RF data including cochannel AM signals received in the samefrequency range and at the same time; and separating each of thecochannel AM signals from other cochannel AM signals of the received RFdata.

In another respect, disclosed herein is a method for transmitting AMsignals that includes providing RF data including cochannel AM signalsfor transmission in the same frequency range and at the same time forreception by a receiver operating in an overloaded signal environment,the receiver configured to separate each of the cochannel AM signalsfrom other cochannel AM signals of the RF data.

In another respect, disclosed herein is a system for communication usingan overloaded signal environment. The system may include: transmitcircuitry configured to provide RF data including cochannel AM signalsfor transmission in the same frequency range and at the same time; andreceive and separation circuitry configured to receive and separate eachof the cochannel AM signals from other cochannel AM signals of the RFdata.

In another respect, disclosed herein is a method for processing AMsignals, that includes receiving RF data that includes cochannel AMsignals. The method may include separating each of the cochannel AMsignals from other cochannel AM signals of the received RF data by: i)providing an initial estimate of (S) of the cochannel AM signals and aninitial estimate of (a) representing amplitudes and phases of thecochannel AM signals; ii) providing at least one additional improvedestimate of (Ŝ^((k))) of the cochannel AM signals based at least in parton a most recent previous estimate (Ŝ^((k−1))) of the cochannel AMsignals, a most recent previous estimate of (a) representing amplitudesand phases of the cochannel AM signals, and the received RF data (r);and iii) repeating step ii) until a specified termination criteria issatisfied.

In another respect, disclosed herein is an AM signal processing systemthat includes receive and separation circuitry coupled to receive RFdata from at least one sensor operating in an overloaded signalenvironment, the received RF data including cochannel AM signalsreceived in the same frequency range and at the same time. The receiveand separation circuitry may be configured to separate each of thecochannel AM signals from other cochannel AM signals of the received RFdata.

In another respect, disclosed herein is an AM signal processing systemthat includes receive and separation circuitry coupled to receive RFdata from a single sensor operating in an overloaded signal environment,the received RF data including cochannel AM signals. The receive andseparation circuitry may be configured to separate each of the cochannelAM signals from other cochannel AM signals of the received RF data by:i) providing an initial estimate of (S) of the cochannel AM signals andan initial estimate of (a) representing amplitudes and phases of thecochannel AM signals; ii) providing at least one additional improvedestimate of (Ŝ^((k))) of the cochannel AM signals based at least in parton a most recent previous estimate (Ŝ^((k−1))) of the cochannel AMsignals, a most recent previous estimate of (a) representing amplitudesand phases of the cochannel AM signals, and received RF data (r); andiii) repeating step ii) until a specified termination criteria issatisfied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates methodology according to one exemplary embodiment ofthe disclosed methods and systems.

FIG. 2 illustrates a system according to one exemplary embodiment of thedisclosed methods and systems.

FIG. 3 illustrates a plot of f(λ) according to one exemplary embodimentof the disclosed methods and systems.

FIG. 4 illustrates a plot of f(λ) according to one exemplary embodimentof the disclosed methods and systems.

FIGS. 5-8 illustrate results of simulations performed according to themethodology of the disclosed methods and systems.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Using the disclosed methods and systems, cochannel AM signals receivedfrom an overloaded signal environment may be separated from each other.In this regard, the disclosed methods and systems may be implemented inone embodiment by modeling a cochannel signal environment to facilitateseparation of cochannel AM signals in an overloaded signal environment.For example, for purposes of illustration, a single sensor receiver mayobserve m temporal samples of n independent AM modulated sourcestransmitting in a common frequency band. In one embodiment of thedisclosed methods and systems, this single sensor situation may berepresented by a uniformly sampled complex baseband received signalmodel of the form where received data (r) is equal to the signal matrix(S) multiplied by the complex vector (a), plus complex noise (n):r=Sa+n  (Equation 1)

where rεC^(m),SεR^(m×n),aεC^(n),nεC^(m). In the model of equation 1, thecolumns of S represent temporally sampled waveforms for each source. Theelements of a are complex scalars representing the amplitude and phaseof each source, and n is a complex noise vector. Using the receivedsignal model of equation 1, the disclosed methods and systems may beimplemented in one embodiment to determine or estimate S given r. Aftersuccessful estimation of S, each column of the estimated S may be passedthrough a conventional AM demodulator to extract the message.

FIG. 1 illustrates exemplary methodology 100 that may be employed usingthe relationship of Equation 1 to separate cochannel AM signals in anoverloaded signal environment according to one exemplary embodiment ofthe disclosed methods and systems. As illustrated in FIG. 1, values of aand S may be initialized in step 110. As will be further described withrespect to the simulations described herein, each of a and S may beinitialized in step 110 as random values, although it is also possiblethat a may be initialized with any other suitable type of value.

In step 120 of FIG. 1, the estimate of S may be improved using the mostrecent previous estimates of a and S. By an improved estimate of S, itis meant that the most recent estimate of S more closely approximatesthe true value of S than a previous estimate of S. In this regard, S maybe estimated the first time in step 120 using the initialized values ofa and S from step 110, and thereafter, as may be necessary, using themost recent previous estimated values of a and S as described furtherbelow. In step 130 of FIG. 1, the estimate of S from step 120 may beconstrained to be real. The results of step 130 (plus other results suchas the previous estimate of S or possibly the current estimate of a) maythen be evaluated against one or more selected termination criteria instep 140. If the termination criteria has been met, then the results maybe accepted in step 160 as shown. However, if the termination criteriahas not been met, a new value of a may be estimated in step 150 usingthe previous estimate of S obtained from step 130. Step 120 may then berepeated as represented by iterative flow path 152, this time using themost recent previous estimate of a from step 150 and the most recentprevious estimate of S from step 130. Using this methodology, steps 120to 150 may be iteratively repeated until one or more selectedtermination criteria are satisfied in step 140, and the results acceptedin step 160.

Referring now to FIG. 1 in more detail, a value for S may be estimatedin step 120 of FIG. 1 using the most recent previous estimate of a andthe previous estimate for S. Since Sa=r has infinite solutions for S,constraints may be imposed to reduce the solution space for S. Forexample, in one embodiment the solution space for S may be reduced byonly considering solutions that are relatively “close” to the mostrecent previous estimate of S, denoted as Ŝ^((k−1)), along with imposinga residual constraint. In this regard, the term “close” refers to thedistance metric in the Frobenius norm sense, where this distance iscalculated by taking the Frobenius norm of the difference between thecurrent estimate, Ŝ^((k)), and the previous estimate, Ŝ^((k−1)). TheFrobenius norm is defined as,${{A}_{F} = \sqrt{\sum\limits_{i = 1}^{m}{\sum\limits_{j = 1}^{n}{a_{ij}}^{2}}}},{{{where}\quad A} \in {C^{mxn}.}}$where AεC^(m×n).

In one exemplary embodiment, a constrained optimization problem may beexpressed as the minimization of the trace of {E^(H)E} as follows$\begin{matrix}{{\min\limits_{E}{{tr}\{ {E^{H}E} \}}}{{{subject}\quad{to}\quad{{r - {( {E + {\hat{S}}^{({k - 1})}} )a}}}^{2}} = \delta}} & ( {{Equation}\quad 2} )\end{matrix}$where E is defined as followsE=Ŝ ^((k)) −Ŝ ^((k−1)),  (Equation 3)

where Ŝ^((k)) is the current estimate of S that minimizes the objectivefunction and satisfies the constraint, where δ is the constraint bound,and where tr{·} is the trace operator. (Note: ∥E∥_(F) ²=tr{E^(H)E}.)

The constraint bound, δ, is a parameter that may be used to define thesquared length of the residual during the estimation of S, i.e., it maybe used to represent the allowable amount of mismatch between theestimates and the observed data. Any suitable constraint boundmethodology may be employed in the practice of the disclosed methods andsystems. For example, a constraint bound value of 0 leads to aleast-squares residual constraint. A constraint bound value may also beselected that is proportional to noise power, e.g., where an estimate ofnoise power is available. Another possible approach to setting aconstraint bound value is to use a methodology that is purely heuristic.

The constrained optimization problem of Equation 2 may be solved usingany suitable methodology. Further information on similar solutionmethodology may be found, for example, in Joel H. Trussel and Mehmet R.Civanlar, The Feasible Solution in Signal Restoration, IEEE Transactionson Acoustics, Speech and Signal Processing, Vol. ASSP-32, No. 2, April1984, pp. 201-212, which is incorporated herein by reference.

The optimization problem of Equation 2 may be solved in one exemplaryembodiment using Lagrangian techniques. In this case, the Lagrangian maybe expressed as $\begin{matrix}{{L( {E,\lambda} )} = {{{tr}\{ {E^{H}E} \}} + {\lambda\{ {{\lbrack {r - {( {E + {\hat{S}}^{({k - 1})}} )a}} \rbrack^{H}\lbrack {r - {( {E + {\hat{S}}^{({k - 1})}} )a}} \rbrack} - \delta} \}}}} & ( {{Equation}\quad 4} )\end{matrix}$

The gradient of the Lagrangian is $\begin{matrix}{{\nabla_{E^{*}}{L( {E,\lambda} )}} = {E + {\lambda\lbrack {{( {E + {\hat{S}}^{({k - 1})}} ){aa}^{H}} - {ra}^{H}} \rbrack}}} & ( {{Equation}\quad 5} )\end{matrix}$

Finding the roots, with respect to E, of the gradient of the Lagrangian,∇_(E).L(E,λ)=0, results in the following expression for E,$\begin{matrix}{E = {( {r - {{\hat{S}}^{({k - 1})}a}} ){{a^{H}( {{aa}^{H} + {\frac{1}{\lambda}I}} )}^{- 1}.}}} & ( {{Equation}\quad 6} )\end{matrix}$

In step 125, the Lagrange multiplier, λ, may be calculated using thesolution for E above and the constraint equation. The expression for theLagrange multiplier may be expressed as below, $\begin{matrix}{{f(\lambda)} = {{( {r - {{\hat{S}}^{({k - 1})}a}} )^{H}{( {r - {{\hat{S}}^{({k - 1})}a}} )\lbrack {1 - {{a^{H}( {{aa}^{H} + {\frac{1}{\lambda}I}} )}^{- 1}a}} \rbrack}^{2}} - {\delta.}}} & ( {{Equation}\quad 7} )\end{matrix}$

A range for the Lagrange multiplier solution may be defined as desiredand an attempt to calculate the solution in the specified range may bemade in step 125. If this attempt to calculate a solution fails, thenthe method may be terminated at step 128 with no available estimate.Alternatively, one or more successively wider ranges for the Lagrangemultiplier solution may be specified, and one or more subsequentattempts made to calculate the solution in the specified successiveranges may be made in step 125 until a solution is found, or until themethod has been performed for all specified successive ranges withoutfinding a solution, at which point the method may be terminated at step128. In determining the root in Eq. 7, the range and resolutionspecified for the bracketed range where the root is assumed to existinfluences the speed of the iteration.

Once the roots of f(λ) are obtained, the correct root may be chosenbased on which root minimizes the objective function, tr{E^(H)E}. Thisroot may be used in Eq. 6 to calculate the projection, E, which may thenbe used to calculate Ŝ^((k)) in step 120.

In step 130, the imaginary portion of Ŝ^((k)) may be discarded and thereal part of Ŝ^((k)) taken to be the current estimate of S at the end ofthe k^(th) iteration of the algorithm, i.e., when the model defines thetemporal samples of each source to be real.

Following step 130, the current estimate of S may be evaluated againstone or more termination criteria in step 140. In this regard, suitabletermination criteria may include any one or more specified criteria thatmay be used at step 140 to determine whether or not the results of step130 should be accepted. In one embodiment, termination criteria mayinclude a specific number of iterations (e.g., iterations of steps 120to 150) that have been performed. For example, steps 120 to 150 mayiteratively performed for a specified given number of iterations,without consideration of other termination criteria. Alternatively, amaximum number of iterations may be specified for terminating theiterations of steps 120 to 150 when the specified maximum number ofiterations have been performed, e.g., when other specified terminationcriteria has not yet been met. Also possible is a specified minimumnumber of iterations that must be performed prior to allowingtermination of the iterations of steps 120 to 150 based on fulfillmentof other specified termination criteria. Other examples of terminationcriteria that may be employed alone or in combination with otherspecified termination criteria include, but are not limited to,requiring a threshold minimum value for length of residual obtained instep 120 (i.e., terminating the iterations when the length of theresidual is less than the specified threshold value), terminating theiterations of steps 120 to 150 when the Lagrange multiplier function(Equation 7) of step 120 has no solution over the specified range forwhich the function is being evaluated, etc. It will be understood thatthe foregoing example of termination criteria are exemplary only, andthat other criteria may be additionally or alternatively employed instep 140.

If the specified termination criteria is satisfied in step 140, then thesteps of method 100 may be concluded, and the results accepted in step160. However, if the termination criteria is not satisfied in step 140,method 100 may proceed to step 150 where a new value of a may beestimated using any suitable method and, for example, using at least aportion of the results previously determined in preceding steps 120 to130. In one embodiment, the elements of a may be jointly estimated instep 150 using a least-squares estimation approach. For example, a newvalue of a may be calculated as described below using r and the mostrecent previous estimate of S obtained in step 130.

In one exemplary embodiment, a least-squares problem may be expressedfor step 150 as an optimization problem with an objective function equalto the squared norm of the residual,J(a)=∥r−Sa∥ ².  (Equation 8)

Thus, the optimization problem is $\min\limits_{a}\quad{{J(a)}.}$The minimum may be found by finding the root of the gradient of theobjective function,${\arg\limits_{a}\quad\bigtriangledown\quad{{\,_{a^{*}}J}(a)}} = 0.$Equating the gradient equal to 0 yields the following expression,∇_(a) ·J(a)=S ^(H) Sa−S ^(H) r=0  (Equation 9)

Therefore, for this exemplary embodiment, the estimate for a in step 150may be expressed asâ=(S ^(H) S)⁻¹ S ^(H) r.  (Equation 10)

The solution may be verified as a minimum by verifying that the Hessianof the objective function is positive definite,∇_(a) ²=∇_(a)∇_(a) ·J(a)=S ^(H) S>0  (Equation 11)

In this case, S^(H)S is positive definite by construction, so thesolution is verified as a minimum.

Once a has been so estimated in step 150, method 100 may proceed viaiterative flow path 152 to step 120, where a new value of S may beestimated using the most recent previous estimate of a from step 150 andthe most recent previous estimate of S from step 130 in a manner aspreviously described. Steps 130 through 140 may then be repeated andmethod 100 terminated if termination criteria is satisfied in step 140,or step 150 may be repeated if termination criteria is not satisfied instep 140. In this manner, steps 150, 120, 125, 130 and 140 may beiteratively repeated until the termination criteria is satisfied in step140, and the method moves to step 160.

Although described herein with respect to a single sensor receiver, itwill be understood that the methodology of the disclosed methods andsystems may be implemented using more than one sensor for separation ofcochannel AM signals in overloaded signal environments.

The disclosed methods and systems may be implemented, for example, aspart of a receiver or transceiver in any manner suitable for achievingthe cochannel signal separation results described elsewhere herein. FIG.2 illustrates one exemplary embodiment of a system 200 as it may beimplemented to receive and to separate cochannel AM signals A and B inan overloaded signal environment. As illustrated in FIG. 2, system 200includes a single sensor in the form of single antenna 220 that iscoupled to receive and separation circuitry 214 that, in this exemplaryembodiment, includes receive path circuitry 210 coupled to signalseparation circuitry 212, it being understood that any otherconfiguration of receive and separation circuitry may be employed thatis suitable for performing one or more of the signal separation tasksdescribed elsewhere herein.

System 200 is illustrated configured as a receive-only system in FIG. 2.However, it will be understood that in other embodiments the disclosedmethods and systems may be alternatively implemented in a systemconfigured as a transceiver. In addition, it is possible that more thanone antenna 220 may be coupled to receive and separation circuitry 214,and/or that antenna 220 may be a single element antenna or an antennaarray. It will also be understood that in other embodiments anoverloaded signal environment may include more than two cochannelsignals.

As illustrated in FIG. 2, antenna 220 is coupled to provide a RF data202 that contains a combination of multiple received AM signals A and Bto receive path circuitry 210. Receive path circuitry 210 (e.g., singlechannel tuner or other suitable circuitry) is configured to process orcondition the received RF data 202 from antenna 220 so as to provideprocessed signal 204 (e.g., as a single tuned channel), that containscombined multiple signals A and B, to signal separation circuitry 212(e.g., implemented as part of a digital signal processor or with othersuitable circuitry). Signal separation circuitry 212 is configured toreceive processed signal 204 from receive path circuitry 210 and toseparate multiple signals A and B. As shown, signal separation circuitry212 is configured to provide multiple differentiated signals A and B(e.g., as respective separate signals 206 and/or 208) to other receiversystem components not shown (e.g., components/circuitry for furtherprocessing, AM demodulation, etc.). It will be understood that separatesignals 206 and 208 may be provided simultaneously by signal separationcircuitry 212, or that only one of differentiated signals 206 or 208 maybe preferentially or selectably provided by signal separation circuitry212.

In the practice of the disclosed methods and systems signal separationcircuitry, such as circuitry 212 of FIG. 2, may be implemented using anycircuit configuration or combination of circuit configurations suitablefor separating two or more combined cochannel AM signals, e.g., receivedfrom one or more sensor/s deployed in an overloaded signal environment.For example, in one embodiment signal separation circuitry may beconfigured in any manner suitable for achieving separation of cochannelAM signals in an overloaded environment using data obtained from one ormore sensors through a method of iterative projections, e.g., usingmethodology such as described in relation to FIG. 1 herein. In oneembodiment, signal separation circuitry 212 of FIG. 2 may be implementedas a digital-signal processor (DSP). Alternatively, or in addition to aDSP, signal separation circuitry 212 may be implemented using any othertype/s of suitable signal processor/s.

It will be understood that the illustrated embodiment of FIG. 2 isexemplary only, and that any other configuration of circuitry and/orsensor/s suitable for accomplishing signal separation according to themethodology disclosed herein is possible. It will also be understoodthat although FIG. 2 illustrates separation of two cochannel AM signalsfrom an overloaded signal environment, the disclosed methods and systemsmay be implemented in overloaded signal environments that include threeor more cochannel AM signals. In FIG. 2, signal separation circuitry 212is shown configured to separate cochannel AM signals A and B and toprovide these as separate output signals 206 and 208. However, it willbe understood that signal separation circuitry may be configured inother embodiments to separate out only one cochannel AM signal receivedfrom an overloaded signal environment including two or more cochannelsignals, e.g., to separate only AM signal A or signal B from theoverloaded signal environment of FIG. 2. In this regard, an overloadedsignal environment may include any given total number of cochannel AMsignals, and the disclosed methods and systems may be implemented in oneembodiment to separate out any number of the cochannel AM signals thatis equal to or less than the total number of cochannel AM signals, asmay be desired or required to meet the needs of a given application. Inone exemplary embodiment, the given total number of cochannel AM signalsmay be characterized as a number of cochannel AM signals having areceived signal strength greater than a given signal strength threshold.

Simulation Results

Following are results of simulations that have been performed byimplementing equations 1-11 in combination with the disclosedmethodology 100 of FIG. 1 for the estimation of AM signals (S) in anoverloaded signal environment. The overloaded signal embodiment employedfor each of the four simulations included one sensor and two cochannelAM signals. The two cochannel signals for each simulation included onerelatively strong cochannel signal (S1) and one relatively weakcochannel signal (S2), although it will be understood that the disclosedmethods and systems may be implemented in overloaded signal environmentshaving two or more AM cochannel signals of equivalent strength.

As described further below, four simulations were performed using randominitial estimates for S, while varying the initial estimate for a. Inthe first simulation, a was initialized to the known true initial phasesof the signals. In the second simulation, a was initialized using randominitial phases for the signals. In the third simulation, a was estimatedusing known incorrect initial phases for the signals. The fourthsimulation was performed using correct initial phases, but including afrequency offset to simulate Doppler frequency shift on the strongsignal (S1). The results of the simulations show that the disclosedmethodology successfully achieves signal separation under the describedconditions with known, random or incorrect phase initialization. Thus,the simulations illustrate how the disclosed methodology may beadvantageously implemented to successfully achieve signal separationeven when initial phases are unknown.

It has been seen in some simulations that initializing with incorrectphases under certain conditions may provide better signal separationthan using random phase initialization. It has been observed that timesfor which this occurs tend to be when the phase of the random values foreach component of a are associated with a small angular separation.

Signal Environment and Signal Generation Parameters for Simulations

Following is a list of signal parameters employed for the simulationsdescribed herein.

Simulated Messages: Simulations were performed with simulated data wherethe messages were constructed by sampling a uniform random distribution.Simulations were also performed using recorded voice data as themessages. In either case, the messages were AM modulated, and zero meanadditive white complex Gaussian noise was added.

Bandwidth Constrained Messages: It has been found that constraining themessage bandwidth to a fraction of the transmission bandwidth mayimprove separation performance.

SNR (Signal to Noise Ratio): SNRs for each of the AM signals were set to20 dB and 15 dB.

Initial phase: The initial phases for the two signals were 30° and 100°.

AM Modulation Index: The modulation index for the simulations was set to0.5, where modulation index is defined as the ratio of the negative ofthe minimum value of the message divided by the amplitude of thecarrier.

Block Size: Block size or number of samples per processing period wasset to 1024 for most cases.

Doppler Frequency: Unless indicated to the contrary, the Dopplerfrequency has been set to 0.

Algorithm Parameter Values for Simulations

The following simulations were carried out using signal environmentsconsisting of two cochannel AM signals plus noise. Therefore, the signalmodel dimension parameter (n) described in relation to Equation 1 wasset to 2. For the simulations, the constraint bound (δ) has been set toa value of 1, which, in this case, is a heuristic value chosen toprovide a solution for S that corresponds to a unit norm residualconstraint. In comparison, choosing a constraint bound value (δ) of zerowould result in a large range of values needed to search over whenfinding the roots for the Lagrange multiplier, λ, in Equation 7. As maybe seen in Equation 7, the value of the constraint bound (δ) acts as aparameter that shifts the overall function up or down. In this regard,when either the length of the residual becomes small, or the constraintbound (δ) is a small value, the root approaches very large values. FIG.3 is a plot of f(λ) for a particular set of data when the constraintbound (δ) is set to 1. FIG. 4 is the same plot zoomed into the range ofthe plot of FIG. 1 where the root occurs.

In the following simulations, the range and resolution defining thebracketed range for locating the root for λ were [−3, 1000] and 5.0,respectively. The selected termination criteria used in the simulationswas a combination of a maximum number of six iterations, and a test forthe existence of a root for the Lagrange multiplier, λ, in Eq. 7 overthe specified range. In this exemplary implementation, the predefinedmaximum number of iterations was chosen to act as an upper bound for themaximum number of iterations allowed, and the test for the existence ofa root for the Lagrange multiplier, λ, was employed to allow thealgorithm to terminate or exit earlier when the length of the residualbecomes small.

Simulation Performance

The performances of four simulation cases are presented in the form ofamplitude versus time plots of the true and estimated signals, withtheir respective phases, passed through a simple AM demodulator. All ofthe following simulations use the parameter values described aboveunless otherwise noted. Also, only a small number of samples wereplotted in each plot so that the tracking of the estimated signalrelative to the true signal may be easily seen.

FIG. 5 illustrates the results of Simulation 1, which was carried outusing correct initial phases. FIG. 6 illustrates the results ofSimulation 2, which was carried out using random initial phases. FIG. 7illustrates the results of Simulation 3, which was carried out usingincorrect initial phases of 0° and 90°. FIG. 8 illustrates the resultsof Simulation 4, which was carried out using correct initial phases, butalso including a frequency offset to simulate a Doppler frequency shiftof 50 Hz on the 20 dB signal at 30° and no Doppler frequency shift onthe weaker signal at 100°.

Each of the four simulations of FIGS. 5-8 illustrate implementation ofthe disclosed methods and systems to separate and estimate both stronger(S1) and weaker (S2) AM cochannel signals. The first three simulationsassumed coherent tuning for both signals. FIG. 8 illustrates animplementation of the disclosed methods and systems in an overloadedsignal environment having a weaker signal with no Doppler shift(coherently tuned) in combination with a stronger signal (having a smallDoppler shift). For this simulation, the block size was reduced from1024 samples to 32 samples with a sampling rate of 10 kHz. In oneexemplary embodiment, the ratio of Doppler frequency to sampling ratemay be used to determine how much phase change exists from sample tosample and to determine performance of the disclosed methods andsystems.

With respect to the simulations described herein, it was noted that thefinal estimate of a may not converge to the true a. In this regard, itmay be reasonable to assume that each component of a may be slightlyperturbed from the true value, since a constraint bound of 1 ignoresalmost all of the noise power. Cases where the phase is initialized toincorrect phases of 0° and 90° may result in estimates that converge toincorrect phases of around 6° and 84°. Even with convergence toincorrect phases, results show that each signal may be estimated. Adecreasing residual with each iteration implies that the estimates of Sand a “fit” the data more and more with each iteration. It will beunderstood that the disclosed methods and systems may be furtherimplemented in other exemplary embodiments with incorporation of Dopplershifts and miss tuning offsets to enhance performance for particularsignal environments.

While the invention may be adaptable to various modifications andalternative forms, specific embodiments have been shown by way ofexample and described herein. However, it should be understood that theinvention is not intended to be limited to the particular formsdisclosed. Rather, the invention is to cover all modifications,equivalents, and alternatives falling within the spirit and scope of theinvention as defined by the appended claims. Moreover, the differentaspects of the disclosed methods and systems may be utilized in variouscombinations and/or independently. Thus the invention is not limited toonly those combinations shown herein, but rather may include othercombinations.

1. A method for processing AM signals, comprising: receiving RF data inan overloaded signal environment, said received RF data comprisingcochannel AM signals received in the same frequency range and at thesame time; and separating each of said cochannel AM signals from othercochannel AM signals of said received RF data.
 2. The method of claim 1,wherein said separating comprises estimating each of said cochannel AMsignals based on said received RF data.
 3. The method of claim 1,wherein said cochannel AM signals are transmitted from geographicallyremote locations; and wherein said method further comprises receivingsaid RF data comprising said cochannel AM signals transmitted fromgeographically remote locations.
 4. The method of claim 3, wherein saidseparating comprises estimating each of said cochannel AM signals basedon said received RF data.
 5. The method of claim 1, wherein saidcochannel AM signals comprise a first cochannel AM signal transmittedfrom a first transmitter and a second cochannel AM signal simultaneouslytransmitted from a second transmitter, said first and secondtransmitters being positioned at geographically remote locations fromeach other; and wherein said method further comprises: providing areceiver; geographically positioning said receiver between said firsttransmitter and said second transmitter; receiving said first cochannelAM signal transmitted by said first transmitter and said secondcochannel AM signal simultaneously transmitted by said secondtransmitter at said receiver while said receiver is geographicallypositioned between said first transmitter and said second transmitter;and separating said received first cochannel AM signal from saidreceived second cochannel AM signal.
 6. The method of claim 5, whereinsaid first and second transmitters are located in separate cities. 7.The method of claim 1, wherein said overloaded signal environment iscreated by intentionally broadcasting said cochannel AM signalssimultaneously in the same frequency range.
 8. The method of claim 7,wherein one of said cochannel AM signals comprises public serviceinformation; and wherein said method further comprises isolating said AMsignal comprising said public service information from other cochannelAM signals of said received RF data.
 9. The method of claim 7, whereinsaid method further comprises selectively isolating a given one of saidcochannel AM signals from other cochannel AM signals of said received RFdata in response to a command specifying the identity of said given oneof said cochannel AM signals.
 10. A method for transmitting AM signals,comprising providing RF data comprising cochannel AM signals fortransmission in the same frequency range and at the same time forreception by a receiver operating in an overloaded signal environment,said receiver configured to separate each of said cochannel AM signalsfrom other cochannel AM signals of said RF data.
 11. The method of claim10, further comprising transmitting said cochannel AM signals fromgeographically remote locations.
 12. The method of claim 10, whereinsaid cochannel AM signals comprise first and second cochannel AMsignals; and wherein said method further comprises transmitting a firstcochannel AM signal from a first transmitter and transmitting a secondcochannel AM signal simultaneously from a second transmitter, said firstand second transmitters being positioned at geographically remotelocations from each other, and said receiver being geographicallypositioned between said first transmitter and said second transmitterfor receiving said first cochannel AM signal transmitted by said firsttransmitter and said second cochannel AM signal simultaneouslytransmitted by said second transmitter.
 13. The method of claim 12,wherein said first and second transmitters are located in separatecities.
 14. The method of claim 10, wherein one of said cochannel AMsignals comprises public service information; and wherein said receiveris further configured to isolate said AM signal comprising said publicservice information from other cochannel AM signals of said received RFdata.
 15. The method of claim 10, wherein said receiver is furtherconfigured to selectively isolate a given one of said cochannel AMsignals from other cochannel AM signals of said received RF data inresponse to a command specifying the identity of said given one of saidcochannel AM signals.
 16. The method of claim 14, wherein said methodfurther comprises receiving said transmitted RF data comprisingcochannel AM signals; and separating each of said cochannel AM signalsfrom other cochannel AM signals of said RF data.
 17. A system forcommunication using an overloaded signal environment, said systemcomprising: transmit circuitry configured to provide RF data comprisingcochannel AM signals for transmission in the same frequency range and atthe same time; and receive and separation circuitry configured toreceive and separate each of said cochannel AM signals from othercochannel AM signals of said RF data.
 18. The system of claim 17,wherein said transmit circuitry is configured to provide said RF datafor transmission of said cochannel AM signals in the same frequencyrange and at the same time from geographically remote locations.
 19. Thesystem of claim 17, wherein said cochannel AM signals comprise first andsecond cochannel AM signals; and wherein said transmit circuitrycomprises a first transmitter configured to transmit a first cochannelAM signal and a second transmitter configured to simultaneously transmita second cochannel AM signal, said first and second transmitters beingpositioned at geographically remote locations from each other; andwherein said receive and separation circuitry is geographicallypositioned between said first transmitter and said second transmitterfor receiving said first cochannel AM signal transmitted by said firsttransmitter and said second cochannel AM signal simultaneouslytransmitted by said second transmitter.
 20. The system of claim 19,wherein said first and second transmitters are located in separatecities.
 21. An AM signal processing system, comprising: receive andseparation circuitry coupled to receive RF data from at least one sensoroperating in an overloaded signal environment, said received RF datacomprising cochannel AM signals received in the same frequency range andat the same time; and wherein said receive and separation circuitry isconfigured to separate each of said cochannel AM signals from othercochannel AM signals of said received RF data.
 22. The system of claim21, wherein said receive and separation circuitry is configured toseparate each of said cochannel AM signals from said other cochannel AMsignals by estimating each of said cochannel AM signals based on saidreceived RF data.
 23. The system of claim 21, wherein said receive andseparation circuitry is coupled to receive RF data comprising cochannelAM signals transmitted from geographically remote locations; and whereinsaid receive and separation circuitry is configured to separate each ofsaid cochannel AM signals from other cochannel AM signals that aretransmitted from geographically remote locations.
 24. The system ofclaim 23, wherein said receive and separation circuitry is configured toseparate each of said cochannel AM signals from said other cochannel AMsignals by estimating each of said cochannel AM signals based on saidreceived RF data.
 25. The system of claim 21, wherein said cochannel AMsignals comprise a first cochannel AM signal transmitted from a firsttransmitter and a second cochannel AM signal simultaneously transmittedfrom a second transmitter, said first and second transmitters beingpositioned at geographically remote locations from each other; whereinsaid receive and separation circuitry is further configured to receivesaid first cochannel AM signal transmitted by said first transmitter andsaid second cochannel AM signal simultaneously transmitted by saidsecond transmitter while said receiver is geographically positionedbetween said first transmitter and said second transmitter; wherein saidreceive and separation circuitry is further configured to separate saidreceived first cochannel AM signal from said received second cochannelAM signal.
 26. The system of claim 21, wherein one of said cochannel AMsignals comprises public service information; and wherein said receiveand separation circuitry is further configured to isolate said AM signalcomprising said public service information from other cochannel AMsignals of said received RF data.
 27. The system of claim 21, whereinsaid receive and separation circuitry is further configured toselectively isolate a given one of said cochannel AM signals from othercochannel AM signals of said received RF data in response to a commandspecifying the identity of said given one of said cochannel AM signals.